The present invention relates to an optical disc formed with phase pits in accordance with an information signal and an optical signal reproducing apparatus for reproducing the information signal recorded in the form of the phase pits, and more particularly to an optical signal reproducing apparatus capable of reproducing phase pit information recorded at a high density which is higher than optical resolution.
Conventionally, in the optical signal reproducing apparatus such as a digital audio disc and a video disc (hereinafter simply referred to as an optical disc apparatus), phase pits complying with an information signal are formed in advance on the signal recording surface of a disc and by utilizing a phenomenon that a light spot for reproduction irradiating on one of the phase pits is reduced in reflection light quantity by diffraction, a decrease in reflection light quantity is detected by a photodetector to reproduce the information signal.
Signal reproduction resolution in the optical disc apparatus is substantially determined by a wavelength .lambda. of the light source for reproduction and a numerical aperture NA of the objective lens and the signal reproduction resolution is limited by a spatial frequency 2NA/.lambda.. Accordingly, in order to realize high-density reproduction, the wavelength .lambda. of the light source of reproduction optics (for example, a semiconductor laser) must be decreased and the numerical aperture NA of the objective lens must be increased.
Contrarily, an optical disc has been proposed in which a layer whose reflection factor changes with temperatures is provided on the optical disc formed with phase pits and a phase pit is read while the reflection factor is partly changed inside a scanning light spot for reproduction.
On the other hand, for focusing error detection or tracking error detection in the optical disc apparatus, an astigmatism method and a three-spot method have generally been employed which have an advantage that a detection system of an optical head can be simplified in construction to attain cost reduction.
In the three-spot method, any offset concomitant with movement of the objective lens and inclination of the optical disc does not occur and therefore this method is also applied to magneto-optical apparatus.
One may refer to, for example, JP-A-3-292632 as a literature relevant to the aforementioned optical disc.